Electronic Packaging and Interconnection: Pb Free Solder Alloys I
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Kazuhiro Nogita, University of Queensland; Mohd Arif Mohd Salleh, Universiti Malaysia Perlis; Dan Li, Beijing University of Technology; David Yan, San Jose State University; Fan-Yi Ouyang, National Tsing Hua University; Patrick Shamberger, Texas A&M University; Tae-Kyu Lee, Cisco Systems; Christopher Gourlay, Imperial College London; Albert T. Wu, National Central University

Monday 2:00 PM
March 20, 2023
Room: Sapphire D
Location: Hilton

Session Chair: Chris Gourlay, Imperial College London; Arif Salleh, Universiti Malaysia Perlis


2:00 PM Introductory Comments

2:05 PM  
Effects of Anisotropic Indium Solder on Cu Diffusion under a Temperature Gradient: Pei Ni Jiang1; 1National Tsing Hua University
    The development of three-dimensional integrated circuit (3D IC) triggers the reduction of solder bump size, causing a larger temperature gradient in the microbumps than flip chip solder joints. Thus, diffusion caused by temperature gradient can be a potential reliability issue in 3D IC packaging. Furthermore, with only a few micrometers of a solder bump, the anisotropic crystal structure of solder material can affect diffusion behavior. In this study, a temperature gradient is established on the symmetrical structure of Cu/In/Cu to investigate how grain orientation of In affects diffusion of Cu under bump metallization. The results shows asymmetrical In-Cu intermetallic compound (IMC) formation are found at two interfaces in some regions; however, symmetrical IMC formation is observed for other regions . The corresponding microstructure evolution and mechanism will be examined and discussed in details in this talk.

2:25 PM  
Characterising the Thermal Expansion Behaviour in In-Sn Superconducting Solder Joints by In-situ Synchrotron Powder X-ray Diffraction: Jiye Zhou1; Xin Fu Tan1; Qinfen Gu2; Stuart McDonald1; Kazuhiro Nogita1; 1The University of Queensland; 2Australian Synchrotron
    Indium based solder alloys are regarded as potential candidates for the next generation of low temperature solder materials, especially for superconducting joints. In this research, the temperature dependent thermal expansion behaviour of two different solder compositions including In-35Sn and In-25.6Sn has been studied using an in-situ synchrotron powder X-ray diffraction (PXRD) method. The variation in lattice parameters in the superconducting phase – β-In3Sn and the Sn-rich phase – γ-InSn4 in the temperature range from 30 °C to 150 °C was characterised. Results revealed that the c-axis of the β-In3Sn unit cell in the In-35Sn alloy exhibited a “smile” parabolic trend as the temperature increases compared to the positive linear trend in In-25.6Sn. The observations are discussed with respect to the equilibrium phase diagram and the proportions and composition of both the γ-InSn4 and β-In3Sn phases present within the alloys.

2:45 PM  
Microalloying Effects of Sb and Ag on the Strain-Rate Sensitivity and Microstructural Evolution of Eutectic SnBi Alloys: Hannah Fowler1; Sukshitha Achar Puttur Lakshminarayana1; Sui Xiong Tay1; Ganesh Subbarayan1; John Blendell1; Carol Handwerker1; 1Purdue University
    Eutectic SnBi solders are reflowed at 180°C, and this limits the heating-induced warpage that occurs during the higher temperature reflow of 240°C required for Sn-Ag-Cu (SAC) solder alloys. In contrast to SAC alloys, however, eutectic SnBi solders are more strain-rate dependent. SnBi performs well during thermal cycling at low strain-rates but performs poorly during the high strain-rates in drop-shock reliability testing. The literature reports that small alloying additions of Sb and Ag can improve drop-shock reliability by increasing the ductility of SnBi alloys and reducing strain-rate sensitivity, without reducing strength. In this presentation, we will report on how different levels of Sb alloying impacts the mechanical properties and microstructural evolution of both eutectic SnBi and eutectic SnBi-1Ag solder alloys on Cu and ENIG (electroless nickel immersion-gold) substrates.

3:05 PM  
The Effects of Sb on the Properties of Hypo-eutectic Sn-Bi Alloys: Xin Tan1; Qichao Hao1; Qinfen Gu2; Stuart McDonald1; Keith Sweatman3; Michael Bermingham1; Kazuhiro Nogita1; 1University of Queensland; 2ANSTO; 3Nihon Superior Co., Ltd
    The research and development of low temperature solders is motivated by environmental, economic and technical needs. Sn-Bi alloys provide the most promising basis for low temperature soldering materials because of their low liquidus temperatures, low costs and non-toxicity. However, the Bi phase is brittle and Sn-Bi solders tend to have low ductility. While the discovery that addition of Sb can improve the ductility of Sn-Bi alloys has attracted strong commercial interest, there is a lack of understanding of the fundamental mechanisms underlying this effect. This study investigates the effects of Sb on the microstructure, crystal structure and solidification behaviour of hypoeutectic Sn-37wt%Bi-xSb alloys, where x is up to 3 wt%. In-situ heating synchrotron PXRD is used to study the time-temperature dependent crystal structure and phase changes. Calculations of phase diagrams (CALPHAD) and density functional theory (DFT) simulations are used to predict the phases and to calculate the energy of formation.

3:25 PM Break

3:45 PM  Invited
Kinetics of the Accumulation of Bismuth at the Anode of a Sn-Bi Based Solder Joint during Current Stressing: Eric Cotts1; Faramarz Hadian1; Javier Flores1; Sitaram Panta1; Mohammed Genanu1; 1Binghamton University
    The evolution of the microstructure of SnBi-based solder joints during current stressing was examined using scanning electron microscopy, and in-situ measurements of electrical resistance. A Bi layer was observed to accumulate at the anode at a linear rate. A correlated increase in the electrical resistance was observed (eventually exceeding the standard JEDEC failure criteria of twenty percent). An expression for the rate of increase of the thickness of the Bi layer was generated from simple diffusion theory, and fit to the data at temperatures between 60 and 125oC, and current densities between 2,000 and 9,000 A/cm2. This expression was related to a specific form of Black's equation for mean time to failure (one inversely proportional to current density, with an apparent activation energy of 1 eV), that produced a good fit to the data for failure times.

4:10 PM  
Surface Precipitation and Growth of Bismuth Particles in Sn-Bi Solder Alloys: John Wu1; Amey Luktuke1; Nikhilesh Chawla1; 1Purdue University
    Bismuth is commonly used as an allowing addition to Pb to lower the solder alloy’s melting temperature and improve the joint strength. Our previous work on SAC has shown that, with small concentrations of Bi, precipitation is driven by a surface diffusion, and the bismuth particles will gather at the exposed surface. We have continued this work on binary Sn-Bi. In this talk we report on a systematic observation of surface precipitation in Sn-3Bi and Sn-7Bi solder alloys. Time-resolved imaging, using scanning electron microscopy was conducted. The precipitation rate was calculated. Correlation of bismuth precipitation to the grain boundaries misorientation angle was conducted and will be discussed.

4:30 PM  
Study of Sn-Bi-In Ternary Solders with Compositions of Lines between Binary Eutectic Points to Ternary Eutectic Points: Hoon Choi1; Hoo-Jeong Lee1; 1Sungkyunkwan University
    In this study, Sn-Bi-In ternary solder for low melting temperature solder is systematically examined. The compositions are selected at two lines from eutectic points of Sn-Bi and that of Sn-In to two ternary eutectic points (56°C and 79°C). For systematic investigation, we performed CALPHAD simulation to obtain phase diagram and thermal information such as pasty zone (liquidus temperature (Tliquidus) – solidus temperature (Tsolidus)). DSC results indicate that several endothermic peaks roughly correspond to the prediction of the phase diagram. Solder balling test shows consistent trends with change of pasty zone predicted by CALPHAD simulation. Microstructures and the intermetallic compounds were examined by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). Shear test and fracture surface analysis disclose the trends of mechanical properties by phase fraction change.